The introduction of electronic flight instrument systems (EFIS) on board aircraft has resulted in a proliferation of flight display features and options. Unfortunately, certain display symbols are inherently incompatible when simultaneously used on the same display. One display system that includes symbols that are incompatible under certain circumstances are the symbols of an electronic landing display. An electronic landing display is a computer-controlled display that indicates to the pilot of an aircraft the relationship between his aircraft and the runway on which the aircraft is to land based on information derived from various aircraft sensors and indicators, such as a radar altimeter, airspeed indicators thrust indicators, etc. Landing displays may include a flight path vector (FPV) symbol and a runway symbol. As the aircraft approaches the runway during a landing, the FPV and runway symbols of the display merge toward one another. One type of such an aircraft landing display system is described in U.S. Pat. No. 4,368,517, entitled "Aircraft Landing Display System" by Peter Lovering.
Landing display symbol incompatibility occurs because as the FPV symbol and the runway symbol merge the symbols tend to occlude one another. Because the symbols occlude one another, the information provided by the symbols may be erroneously interpreted by a pilot. This possibility becomes greater when the runway symbol rises and includes a radar altitude value in digital form that rises with the runway symbol. Such a display is presently scheduled for inclusion in the electronic flight instrument system (EFIS) designed for the 747-400 airplane produced by The Boeing Company, Seattle, Washington. In this particular instrument, the FPV symbol occludes the digital altitude value when the rising runway symbol and the FPV symbol overlap.
In the past, aircraft instrument systems have often included some provision for solving this incompatibility problem. For example, the flight display system incorporated on the A320 produced by the European Commercial Airplane Manufacturing Consortium, commonly referred to as Airbus, includes a rising runway symbol combined with a radar altitude value and a flight path vector or flight path angle symbol. The Airbus approach to solving the occlusion problem is to allow one symbol to disappear behind the other, somewhat similar to the way personal computer (PC) graphics provide overlapping displays. (The overlapping display solution in the personal computer environment is commonly referred to as a "window display". In such displays, a "window" overlaps the main displays; i.e., the main display disappears behind the window display.)
Flight management displays, including means to eliminate undersired symbols, are also described in U.S. Pat. No. 3,668,622, entitled "Flight Management Display" by James R. Ganett et al. and in U.S. Pat. No. 4,247,843, entitled "Aircraft Flight Instrument Display System" by Harry Miller et al.
While solutions of the type incorporated in the Airbus A320 flight display instrument and described in the foregoing patents reduce the symbol incompatibility problem and/or eliminate undesired symbols, they are not as satisfactory as desired when applied to a landing system. As will be better understood from the following description, the present invention is directed to solving the symbol incompatibility problem in an aircraft landing display system by removing one or the other of the merging symbols depending upon the flight status of the aircraft.